Stretching machines are known essentially formed by a robust bench upon which one or more motorised work heads are mounted through which the hides to treat are made to advance, driven in a substantially horizontal direction by reciprocatingly facing flexible mat pairs.
Each work head is formed by at least one pair of beating masses or bodies to which plates or dies of appropriately shaped tools are connected, adapted to reciprocatingly interact in a manner to stretch the fibres of the interposed hides. To this end, at least one of the plates is provided with a relative movement towards and away with respect to the other plate in a work plane substantially perpendicular to that of advancement of the hides.
Examples of stretching machines of the above indicated type are illustrated, for example, in the patents IT-A-883 012, FR-A-2 325 718, GB-A-809 179, GB-A-1 016 191, and EP-A-0 368 233.
In such known machines the alternative movements of the plates are generally obtained by means of connecting rod and crank mechanisms, or by means of eccentrics or cams placed in rotation by a driving shaft connected to a motor which is independent or common to all the heads.
A recognised inconvenience of these known machines consists in the notable vibrations created by the oscillating movements of the bodies in motion. Such vibrations damage the structure and are sources of excessive sonorous emissions.
To this end, on the principal shaft of the machine or of each head there is normally applied a fly-wheel mass which serves to confer uniformity to the motion. However, the presence of such fly-wheel mass cannot completely eliminate the vibrations created by the alternative motion of the plates. Such residual vibrations accelerate fatigue wear to the structure with consequent reduction in reliability and in the average life of the machine as a whole.
In order to further limit the vibrations it has been proposed to provide machines with multiple working heads staggered in a horizontal plane, reciprocatingly connected by appropriate synchronisation means adapted to maintain the oscillating adjacent plates in opposite phase. This solution, described and claimed in the patent EP-A-0 504 579 which is incorporated herein by reference, does not allow however to eliminate the reaction moments created by the inertial forces of the adjacent plates and therefore it does not completely solve the problems of the vibrations.
Another inconvenience of the known past machines is constituted by their lack of flexibility, in that the number of initial heads and the dimensions of the supporting structure may not be successively modified for increasing the potentiality of the machine. Moreover, each head carries out only one treatment of the hides at a time and therefore in order to increase the productivity of the machine the number of heads and therefore its cost must be increased.
Another inconvenience of known stretching machines is constituted by their notable dimensions and weights which, in addition to limiting the maximum working frequency, brings about a major production cost and elevated consumption.